The long history of earth is one of vast geological tumult and change. As new continents have been formed, large land masses have broken apart or collided and combined with great pressures exerted on tectonic plates. This great pressure and movement on these tectonic plates over time has created many earthquakes and seismic fault lines to appear.
Measuring seismic waves with surface instrumentation is many centuries old. As long ago as 132 AD, Zhang Heng of China's Han dynasty invented a functional seismic detector. This was a large bronze vessel, about 2 meters in diameter. At eight positions around the top of the vessel were dragon's heads holding bronze balls. When an earthquake occurred, the internal mechanisms would react to the direction of the seismic waves and cause one of the mouths to open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake.
Small movements of the earth can foreshadow larger events to follow. Typically, earth movements have been detected by a variety of sensors placed on or near the surface. Currently, electronic sensors also mounted at the surface are used to provide broadband radio-frequency detection covering a wide range of frequencies produced by earth movements. Some seismometers can measure motions with frequencies from 500 Hz to 0.00118 Hz (1/500=0.002 seconds per cycle, to 1/0.00118=850 seconds per cycle). Greater sensitivity can be obtained if underground devices directly measure earth movements and frequencies that are too weak to be detected above noise distortion at the surface.
If one looks to the history of large earthquakes over our long history on earth, it becomes alarmingly clear that the number of large quakes has drastically gone up over the last 15 years. We are living in a time of large-scale quakes, and because of overpopulation and lax earthquake-resistant building codes, significant populations live in areas where survival is influenced by earthquake dangers.
Down-hole instrumentation for seismological purposes is sometimes used by creating a borehole from the surface. For example, triaxial sensors have been employed down a borehole to measure ground motion and the potential for seismic amplification at surface structures, such as bridges. Such seismic engineering practice requires a costly and complex drilling operation to remove the earth to create multiple smooth boreholes and then lowering the instrumentation package from the surface down each such borehole to the measurement locations.